The present invention relates to a method for preparing an anhydrous ferric chloride from a dilute ferrous or ferrous-ferric chloride solution.
Ferric chloride FeCl.sub.3 is conventionally prepared in an anhydrous form by allowing a chlorine gas to affect glowing iron. As a solution it is in turn obtained by dissolving an iron oxide or carbonate or a metallic iron in a hydrochloric acid or nitrohydrochloric acid. When crystallizing from water, the ferric chloride forms various hydrates, such as a hexahydrate FeCl.sub.3 . 6 H.sub.2 O), i.e., the so-called yellow iron chloride, known as a commercial product.
Methods are known, in which the ferrous chloride FeCl.sub.2 acting as a reactant is oxidated into ferric chloride. FI Patent publication No. 77006 (int. cl. CO1G 49/00) describes a method for partially oxidating FeCl.sub.2 into FeCl.sub.3. In the method, a temperature of 350.degree. C.-675.degree. C. is maintained in a tubular reaction area, and a filling layer moving downwards contains a solid ferrous chloride and carbon, whereas oxygen gas is passed from below upwards.
Several ways are known for converting ferrous chloride into ferric chloride, e.g. for using chlorine (FI Application Nos. 892059, 892060 and 892061).
A method is also known for converting a concentrated ferrous solution into ferric chloride, e.g. by means of oxygen gas (U.S. Pat. No. 4,248,851).
Ferric chloride is used, e.g., for purifying water as well as an oxidation agent in industry, e.g. in the color industry, as a catalyst, e.g. in Friedel-Crafts syntheses.
The ferric chloride form used most commonly is a solution, but in some specific application conditions, an anhydrous crystalline salt is required.
For example, when preparing conductive polymers, anhydrous ferric chloride (FeCl.sub.3) is used as an oxidation catalyst, which is reduced in the process partially a bivalent ferrous chloride (FeCl.sub.2).
The ferric chloride acts in the reaction as an oxidant, receiving 2 electrons per 1 mol of monomer and simultaneously reducing into a ferrous form. The releasing protons and the chloride are combined to form HCl. Water and possibly alcohol is used for washing the polymer, and the washing water obtained is a dilute ferrous-ferric chloride solution containing hydrochloric acid and organic compounds.
To date, it has not been possible to prepare anhydrous FeCl.sub.3, since ferric chloride easily decomposes into oxychloride and hydrochloric acid.